A close cousin of assimilation is the "outgroup homogeneity effect." In the language of social psychology, an "ingroup" is a group to which someone belongs, and an "outgroup" is a group to which the person does not belong (hence, one person's ingroup may be another person's outgroup, and vice versa). Research on the outgroup homogeneity effect has found that when it comes to attitudes, values, personality traits, and other characteristics, people tend to see outgroup members as more alike than ingroup members. As a result, outgroup members are at risk of being seen as interchangeable or expendable, and they are more likely to be stereotyped. This perception of sameness holds true regardless of whether the outgroup is another race, religion, nationality, college major, or other naturally occurring group (Linville, 1998).

In one of the first studies to document the outgroup homogeneity effect, Princeton University researchers asked students in four different "eating clubs" to rate members of their own group and members of three other groups on personality dimensions such as introverted-extroverted and arrogant-humble (Jones, Wood, & Quattrone, 1981). The results showed that students tended to rate members of their own group as more varied in personality than members of the outgroup -- regardless of which group students were in. Later research on outgroup homogeneity found that the effect is strongest when the ingroup and outgroup are enduring, real-life groups (rather than groups created artificially in laboratory experiments), and when the ingroup is large (Mullen & Hu, 1989). If the ingroup is small and the attributes in question are important to its identity, the outgroup homogeneity effect may disappear or even reverse (Simon, 1992; Simon & Pettigrew, 1990).

Why are outgroups generally seen as more homogeneous than ingroups? One possible reason is that people usually have less contact with outgroup members than ingroup members, and indeed, there is good evidence for this explanation (Islam & Hewstone, 1993; Linville & Fischer, 1993). But contact alone cannot explain the outgroup homogeneity effect, because some studies have found that the effect is unrelated to the number of ingroup or outgroup members a person knows (e.g., Jones, Wood, & Quattrone, 1981). Furthermore, perceptions of outgroup homogeneity are sometimes found among groups that have extensive contact with each other, such as females and males (Park & Rothbart, 1982; Park & Judd, 1990). When men complain that "women are all alike" and women complain that "men are all alike," their charges rarely stem from a lack of contact.

The best explanation is that a variety of factors produce the outgroup homogeneity effect. In addition to the fact that people usually have more contact with ingroup members, they tend to organize and recall information about ingroups in terms of persons rather than abstract characteristics (Ostrom, Carpenter, Sedikides, & Li, 1993; Park & Judd, 1990). In many cases, people are also more motivated to make distinctions among ingroup members with whom they will have future contact (Linville, 1998). When these factors operate together, the end result is often an ingroup that appears to have a diverse assortment of individuals, and an outgroup that appears relatively homogeneous and undifferentiated.